WASHINGTON — New research from Canada has found that roughly 1 in 5 adolescents has probably suffered a traumatic brain injury — a figure that suggests severe concussion among children and adolescents may be far more common than has been estimated. The new study also hints at a troubling link between a history of traumatic brain injury and poorer grades, underage drinking and use of illicit drugs.

In Ontario, Canada, 62 percent of students in grades seven through 12 anonymously completed a computerized questionnaire administered during the school day, which gauged their drug and alcohol consumption patterns and a wide range of health-related behaviors. Among the questions, students were asked if they had ever been knocked unconscious for five minutes or more or had been kept overnight in a hospital following a blow to the head. The survey also asked students to indicate if such an incident had occurred in the last year.

The results, published Tuesday as a “research letter” in the Journal of the American Medical Association, were startling. A total of 20.2 percent of respondents reported that at some point in their average 15 years of life, they had either been hospitalized overnight after a blow to the head or had been knocked unconscious for more than five minutes. And 5.6 percent — more than 1 in 20 students — said they had suffered such an injury within the last year.

Concussions are often diagnosed after blows to the head far milder than those causing loss of consciousness or hospitalization. So this study probably yields a very conservative estimate of brain injury among children. Though it’s not known how widespread traumatic brain injuries are among American kids, almost half a million children under 15 are brought to hospital emergency departments each year in the United States to assess signs of such an injury.

For as many as 15 percent of patients with traumatic brain injury, the Centers for Disease Control and Prevention estimates that symptoms such as headaches, dizziness, concentration difficulties and mood changes may persist for more than a few days or weeks. But the Canadian researchers found evidence suggesting that a history of such injuries may also suppress academic performance and increase likelihood of drug or alcohol use.

Among students who reported receiving poor grades in school, the likelihood of having a history of traumatic brain injury was almost four times greater than among those who received high grades. Those who reported the most frequent use of alcohol were about twice as likely to have a history of traumatic brain injury as were those who could recall no such history. And those who reported a severe blow to the head in the last year were seven times more likely to report such alcohol use than were those with no history of traumatic brain injury. Frequent cannabis use was more than four times more common among those with a recent history of traumatic brain injury than among those who had never been concussed.

That relationship between traumatic brain injury and poor grades and alcohol and cannabis use “needs further investigation,” the researchers wrote. Such findings do not reveal whether a history of concussion puts children at higher risk of drug and alcohol abuse and poor grades, or whether children already at risk for those outcomes might be more prone to head injury.